Polycrystalline diamond substrate and process for producing the same

Abstract: A diamond substrate (1) having a smooth surface, including a polycrystalline diamond film having a surface with a pit (2), and an insulating material other than diamond, which occupies the pit.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a polycrystalline diamond substrate, more particularly to a polycrystalline diamond substrate having a smooth surface and a process for producing such a polycrystalline diamond substrate. Related Background Art Diamond has the highest hardness among all of the solid materials which are present on the earth, is electrically an insulator and has the highest thermal conductivity within a temperature range of 30 to 650 °C. In addition, diamond optically has an excellent light transmission characteristic in a wide wavelength range except for a certain part of an infrared region. Therefore, diamond having such outstanding characteristics is expected to be utilized in various technical or scientific fields. Further, diamond has the highest sound velocity among all of the substances, and is stable thermally and chemically. Therefore, by utilizing such characteristics of diamond, for example, an application of diamond to devices such as surface acoustic wave (SAW) devices has been investigated. A surface acoustic wave device utilizing diamond can be used in an extremely high frequency region, e.g., by using a diamond film as a substrate, as disclosed in Japanese Laid-Open Patent Application Nos. Sho 64-62911 (No. 62911/1989) and Hei 3-198412 (No. 198412/1991). The surface acoustic wave device using a diamond film as a substrate may be fabricated in the following manner. First, a diamond film is formed on a substrate of Si, etc., by vapor-phase synthesis. Next, the surface of the vapor-phase synthesized diamond film is polished to be smooth. Further, the vapor-phase synthesized diamond film having the smooth surface is used as a substrate, and an electrode is formed thereon by microfabrication. As described above, when a surface acoustic wave device using a diamond film as a substrate is intended to be fabricated, the surface of the diamond film formed by vapor-phase synthesis is polished before an electrode is formed on the diamond film. However, since the diamond has a high hardness, polishing of the surface thereof is not sufficient in some cases, and therefore some scratches caused by the polishing can be left on the surface. Further, since the vapor-phase synthesized diamond film is polycrystalline, dropout of some particles may occur due to the polishing treatment, or an unfilled or unoccupied portion remaining in the inside of the diamond film may appear on the surface thereof as a hole due to the polishing. Therefore, in general, some pits or concavities having a dimension-of up to few microns still remain on the surface of the vapor-phase synthesized diamond film even after the polishing thereof. Accordingly, in the prior art, there has been posed a problem that when a fine electrode or line having a dimension of submicron to several microns is formed on the vapor-phase synthesized diamond substrate having such a surface which is not sufficiently smooth, the fine electrode or line is liable to be cut or broken. IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 34, NO. 3, August 1991 discloses a method of improving the surface texture of polycrystalline diamond wherein amorphous carbon is deposited on polycrystalline diamond. U.S. Patent No. 5,036,373 discloses a diamond electric device comprising: a substrate, a diamond film formed on the substrate, and an insulator filling the recesses of the diamond film. In this reference, recesses of columnar diamond crystals are filled with and insulated by an insulating material such as a photoresist or a molten glass. D3 (JP-A-61 210 518) describes a diamond-like amorphous carbon film having a specific Vickers hardness and a specific resistivity. SUMMARY OF THE INVENTION An object of the present invention is, in view of the above-mentioned problems encountered in the prior art, to provide a polycrystalline diamond substrate having a smooth surface. The object is achieved with a diamond substrate as defined in claim 1 and a process as defined in claim 12. Preferred embodiments are defined in the subclaims. The above-mentioned polycrystalline diamond substrate according to the present invention has a smooth surface of which a pit (or concavity) is occupied or filled with an insulating material other than diamond. Accordingly, even when an electrode and/or line is formed by microfabrication on the above-mentioned polycrystalline diamond substrate of which the surface is sufficiently smooth, substantially no breakage of the electrode or line occurs. In addition, in the process for producing a diamond substrate according to the present invention, an insulating film comprising a material other than diamond is formed on a polycrystalline diamond film of which the surface has a pit, and then the insulating film is polished so that the surface of the diamond film is exposed. Accordingly, the pit of the polycrystalline diamond film surface is occupied or filled with the insulating material other than diamond, whereby the surface of the diamond film becomes smooth. Further objects and advantages of the present invention will be apparent from the description of the preferred embodiments with the accompanying drawings. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view showing a polycrystalline diamond substrate according to an embodiment of the present invention. Fig. 2 is a schematic chart showing typical Raman spectra of amorphous carbon (a), graphite (b), and diamond (c), respectively. Figs. 3 to 5 are schematic sectional views for illustrating a process for producing a polycrystalline diamond substrate according to an embodiment of the present invention. Fig. 3 shows a state wherein a polycrystalline diamond film is formed on a predetermined substrate. Fig. 4 shows a state wherein an insulating film comprising a material other than diamond is formed on the polycrystalline diamond film. Fig. 5 shows a state of the polycrystalline diamond film after the insulating film is polished so that the surface of the polycrystalline diamond film is exposed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinbelow, preferred embodiments of the present invention will be described with reference to the accompanying drawings as desired. (Polycrystalline diamond substrate) Fig. 1 is a schematic side sectional view showing a polycrystalline diamond substrate according to an embodiment of the present invention. Referring to Fig. 1, the polycrystalline diamond substrate comprises a polycrystalline diamond film 1 and an insulating material 2 located in a pit (or pits) of the surface of the diamond substrate 1. The placement of the insulating material 2 in the pit of the surface of the diamond substrate 1 may for example be confirmed by distinguishing the diamond film 1 from the insulating material 2 by a secondary-electron reflection image formed by a scanning electron microscope (SEM), or by a surface analysis conducted by micro Auger electron spectroscopy. The polycrystalline diamond substrate according to the present invention has a smooth surface. The surface smoothness (or surface roughness) of the diamond substrate according to the present invention may preferably be 50 nm (500 Å) or below, more preferably 10 nm (100 Å) or below, in terms of R max (JIS (Japanese Industrial Standard) B 0601-1970; Osamu Taniguchi, "KIKAI KEISOKU (Mechanical Metrology)", pp. 77-80, Youkendo, 1974"), based on a tracer method. The surface smoothness of the diamond substrate according to the present invention may also be evaluated in the following manner. That is, an Al film having a thickness of 0.1 µm is formed on the above-mentioned diamond substrate to be evaluated by a sputtering method, and then the Al film is patterned by photolithography and reactive ion etching (RIE), thereby to form 100 Al lines each having a width of 1 µm and a length of 500 µm. When the conductivities of the 100 Al lines thus formed are examined by means of a prober, in the diamond substrate according to the present invention, the number of broken or cut lines in the above-mentioned 100 Al lines may preferably be 10 or below, more preferably 5 or below (particularly preferably 2 or below). (Polycrystalline diamond) In the present invention, a method of forming a polycrystalline diamond film 1 is not particularly restricted. In view of easiness of the formation of a film having a desired thickness, the polycrystalline diamond film 1 may preferably be formed by vapor-phase synthesis or vapor deposition. In view of easiness of the formation of a film having a desired characteristic, the polycrystalline diamond film 1 may preferably be formed by a CVD (chemical vapor deposition) process. (Insulating material) As the above-mentioned insulating material 2 for filling the pit of the surface of the polycrystalline diamond film 1, an insulating material other than diamond is used. The resistivity of the insulating material 2 (measured by a four-probe method) may preferably be 10 6 Ω·cm or higher, more preferably 10 8 Ω·cm or higher. In view of the heat resistance of the insulating material 2, the insulating material 2 may preferably be an inorganic material. In addition, the insulating material 2 may preferably have a softening point of 300 °C or higher (more preferably 500 °C or higher). In view of easiness of the deposition of the insulating material 2 on (or into) the pit of the surface of the polycrystalline diamond film 1, the insulating material 2 may preferably be a material which can be deposited by vapor deposition technique. More specifically, preferred examples of the insulating material 2 may include: oxides such as SiO 2 , YSZ (Y 2 O 3 (yttria)-stabilized ZrO 2 (zirconia)), Al 2 O 3 , and ZnO; carbides such as SiC and TiC; nitrides such as Si 3 N 4 and AlN; and DLC (diamond-like carbon). (Diamond-like carbon) In the above-mentioned specific insulating materials, DLC has features such that it is an amorphous material having a high hardness and has an excellent stability, and practically, it is not necessary to consider element diffusion or/and reaction thereof to diamond, since the DLC comprises carbon atoms which are the same as those constituting diamond. Therefore, the DLC may particularly preferably be used as an insulating material 2 for filling the pit of the diamond substrate 1. The above-mentioned DLC (also called "i-carbon" or "amorphous carbon") is a material having the following properties. (1) In general, the DLC comprises hydrogen atoms in addition to carbon atoms. In this case, the mole of hydrogen atoms contained in the DLC may preferably be smaller than the mole of carbon atoms contained in the DLC. (2) The DLC has an amorphous crystalline state. The DLC, diamond, and graphite may mutually be distinguished from each other by means of a Raman spectroscopic method. Typical Raman spectra of DLC (or amorphous carbon) (a), graphite (b), and diamond (c) are shown in Fig. 2. As shown in Fig. 2, a sharp peak appears at 1332 cm -1 (based on sp 3 C-C bond) in the spectrum of diamond (c), and a sharp peak appears at 1580 cm -1 (based on sp 2 C-C bond) in the spectrum of graphite (b), whereas broad peaks appear at 1360 cm -1 and 1600 cm -1 in the spectrum of DLC (c). (3) The DLC has a higher hardness than that of an ordinary metal. More specifically, the DLC to be used in the present invention may preferably have a Vickers hardness Hv of about 1,000 to 5,000, while diamond usually has a Vickers hardness Hv of about 10,000. The DLC having the above-mentioned characteristics may be obtained under the same CVD condition as that for diamond formation described hereinafter, while decreasing the temperature of a substrate on which the DLC is to be deposited (for example, the substrate temperature may preferably be about 100 °C). With respect to more detail of DLC, for example, Akio Hiraki and Hiroshi Kawarada, "TANSO (Carbon)" published by The Carbon Society of Japan, Tokyo, 1987 (No. 128), p.41" may be referred to. Figs. 3 to 5 are schematic side sectional views for illustrating a process for producing a polycrystalline diamond substrate according to an embodiment of the present invention. Referring to Fig. 3, first, a polycrystalline diamond film 1 is formed on a predetermined substrate (e.g., an Si substrate) 3. The diamond film 1 may preferably have a thickness of 10 µm or larger (more preferably 25 µm or larger). Next, as desired, the thus formed diamond film 1 is polished, e.g., by using a diamond abrasive. The diamond film 1 after the polishing may preferably have a thickness of 5 µm or larger (more preferably 20 µm or larger). When the thickness of the diamond film 1 before the polishing is denoted by t 1 and the thickness of the diamond film 1 after the polishing is denoted by t 2 , in general, the value of (t 1 - t 2 ) may preferably be 2 µm or larger (more preferably, 5 µm or larger). As described hereinabove, after such polishing, the diamond film 1 usually has a pit 4 on the surface thereof (Fig. 3). Next, referring to Fig. 4, an insulating film 12 comprising an insulating material 2 other than diamond is formed on the diamond film 1, e.g., by vapor-phase synthesis. The insulating film 12 thus formed may preferably have a thickness of about 0.1 to 100 µm (more preferably about 1 to 10 µm.) Further, referring to Fig. 5, the surface of the diamond film 1 is exposed by polishing the insulating film 12 which has been formed on the diamond film 1. In general, such polishing may be performed by using various abrasives (or abrasive grains). The abrasive to be usable for such a purpose is not particularly limited as long as the abrasive has a hardness higher than or equal to that of the insulating film 12, and comprises a material having a lower hardness than that of diamond (that is, a material other than diamond). More specifically, for example, it is possible to use an abrasive (for example, conventional sand paper) comprising SiC, Al 2 O 3 , etc. The exposure of the surface of the diamond film 1 due to the polishing may for example be confirmed by use of an optical microscope or surface-enhanced Raman spectroscopy. At the time of the polishing as described above, in a case where an abrasive comprising a material other than diamond is used, the diamond film 1 itself is not polished since the diamond film 1 has a higher hardness than that of the abrasive to be used for the grinding. As a result, the pit 4 of the surface of the diamond film 1 is filled or occupied with the insulating material 2 other than diamond, whereby a diamond substrate (as shown in Fig. 5) having a smooth surface may be obtained. The present invention will be described in more detail with reference to specific Examples. Examples Polycrystalline diamond substrates of which the surfaces were smooth were produced by vapor-phase synthesis in the following manner. (1) Referring to Fig. 3, a 1.0 mm-thick Si substrate 3 (10 mm × 10 mm square) the surface of which had been polished to be smooth by electrolytic polishing was placed in a plasma CVD apparatus. A reaction chamber of the CVD apparatus was evacuated, and a mixture gas of H 2 and CH 4 (H 2 : CH 4 = 200 : 1 in terms of a volume ratio; hereinafter, gas ratios are expressed by volume ratios) was introduced into the reaction chamber. Plasma CVD was conducted to form a 50 µm-thick polycrystalline diamond film 1 on the Si substrate 3 under the conditions that the pressure of the reaction chamber was about 53 mbar (40 Torr), the temperature of the substrate 3 was 850 °C, and the power of microwave was 400 W. (2) Next, the thus formed diamond film 1 was subjected to mechanical polishing by using an electrodeposited diamond polishing stone, and the diamond film 1 was polished so as to provide a film thickness of 30 µm. (3) On the thus polished diamond film 1, a film 12 of an insulating material (A) as shown in the following Table 1 was formed so as to provide a film thickness of 10 µm by a film formation process (B) as shown in the following Table 1. The film formation process (B) shown in the above Table 1 was conducted under the following conditions. <Sputtering> (with respect to SiO 2 , ZnO and Al 2 O 3 ) RF power: 200 W Substrate temperature: 200 °C Sputtering gas: Ar containing 20 % of O 2 Gas pressure: (100 mTorr) 0.133 mbar <Thermal CVD> (with respect to SiC) Substrate temperature: 1000 °C Gas: 50 % of propane + 50 % of silane Gas pressure: (760 Torr) 1,013 bar <MOCVD> (with respect to AlN) Substrate temperature: 900 °C Gas: 60 % of trimethyl aluminum + 40 % of ammonia Gas pressure: (760 Torr) 1,013 bar <Plasma CVD> (with respect to DLC) RF power: 200 W Substrate temperature: 100 °C Gas: Methane Gas pressure: (5 mTorr) 6.7 · 10 -3 mbar (4) Next, the above-mentioned insulating film 12 of the insulating material (A) was polished by using an abrasive grain (C) as shown in the above Table 1, until the surface of the diamond film 1 was exposed. The exposure of the surface of the diamond film 1 was confirmed by using an optical microscope. In this case, as the SiC abrasive grain, commercially available sand paper was used. Further, as the Al 2 O 3 abrasive grain, commercially available Al 2 O 3 fine powder was used, and buff polishing was conducted. At the time of the above polishing, the diamond film 1 was not polished since the diamond film 1 itself had a higher hardness than that of the abrasive grain (C) used for the polishing. Based on the above treatment, the pit 4 of the surface of the diamond film 1 was filled with the insulating material (A) other than diamond, and six kinds of diamond substrates (Examples 1 to 6) having a smooth surface were obtained (Fig. 5). On the vapor-phase synthesized diamond substrate obtained above, Al lines were formed in the following manner. (5) First, an Al film having a film thickness of 0.1 µm was formed by sputtering on the above-mentioned diamond substrate under the following conditions. DC power: 3 KW Substrate temperature: room temperature Sputtering gas: Ar Gas pressure: (8 mTorr) 10.7 x 10 -3 mbar (6) Next, the thus formed Al film was patterned by photolithography and reactive ion etching (RIE), thereby to form 100 Al lines each having a width of 1 µm, and a length of 500 µm in the samples obtained in the above Examples 1 to 6. <RIE conditions> Gas: CCl 4 (50%) + Cl 2 (50%) Gas pressure: (40 mTorr) 53 . 10 -3 mbar RF power: 200 W Substrate temperature: 30 °C (7) Conductivities of the 100 Al lines thus formed were examined by use of a prober, and the number of broken lines in the above-mentioned 100 Al lines was measured. The results are also shown in the above Table 1. On the other hand, six species of vapor-phase synthesized diamond substrates were prepared in the same manner as in the above Examples 1 to 6, except that the above-mentioned steps of (3) and (4) were not conducted. Then, 100 Al lines were formed on the thus prepared substrates, and the number of broken lines was measured in the same manner as described above. The results are shown in the above Table 1. As shown in the above Table 1, since the diamond substrate according to the present invention had a high surface smoothness, even when fine lines or electrodes were formed on the diamond substrate, breakage of the line was effectively suppressed. As described hereinabove, according to the present invention, a polycrystalline diamond substrate having a smooth surface may be obtained. Even when an electrode or line is formed by microfabrication on the polycrystalline diamond substrate according to the present invention, breakage of the electrode or line is prevented. Accordingly, the polycrystalline diamond substrate according to the present invention may effectively be used for, e.g., a substrate for a device such as a surface acoustic wave device. In addition, the polycrystalline diamond substrate according to the present invention may preferably be used as a substrate for mounting a semiconductor device such as multi chip module (MCM). Moreover, the diamond substrate according to the present invention may also be utilized for various semiconductor devices (such as transistors and diodes) in which diamond is used for a substrate.

Claims: A diamond substrate having a mechanically polished surface, comprising: a polycrystalline diamond film having a surface with a pit, and an insulating material other than diamond, which occupies the pit, at least a portion of the surface of the polycrystalline diamond film being exposed from the insulating material. A diamond substrate according to Claim 1, which has a surface smoothness of 50 nm (500 Å) or below in terms of Rmax. A diamond substrate according to Claim 1, wherein the polycrystalline diamond film is a film formed by vapor-phase synthesis. A diamond substrate according to Claim 3, wherein the polycrystalline diamond film is a film formed by chemical vapor deposition. A diamond substrate according to Claim 1, wherein the insulating material has a resistivity of 10 6 Ωcm or higher. A diamond substrate according to Claim 1, wherein the insulating material comprises an inorganic material. A diamond substrate according to Claim 1, wherein the insulating material is a material which is capable of forming a film by vapor deposition. A diamond substrate according to Claim 1, wherein the insulating material is a material selected from the group consisting of oxides, carbides and nitrides. A diamond substrate according to Claim 8, wherein the insulating material is a material selected from the group consisting of SiO 2 , Y 2 O 3 -stabilized ZrO 2 , Al 2 O 3 , ZnO, SiC, TiC, Si 3 N 4 and AlN. A diamond substrate according to Claim 1, wherein the insulating material comprises diamond-like carbon. A diamond substrate according to Claim 10, wherein the diamond-like carbon has a Vickers hardness of about 1,000 to 5,000. A process for producing a diamond substrate, comprising: forming an insulating film comprising a material other than diamond on a polycrystalline diamond film disposed on a substrate; and mechanically polishing the insulating film to expose a surface of the polycrystalline diamond film, thereby to provide a smooth surface, wherein the abrasive to be used in the mechanical polishing has a hardness higher than that or equal to that of the insulating film and lower than that of diamond.

Inventors: HACHIGO AKIHIRO, HIGAKI KENJIRO, NAKAHATA HIDEAKI, SHIKATA SHINICHI, HACHIGO, AKIHIRO, HIGAKI, KENJIRO, NAKAHATA, HIDEAKI, SHIKATA, SHINICHI, HACHIGO, AKIHIRO ITAMI WORKS OF SUMITOMO, HIGAKI, KENJIRO ITAMI WORKS OF SUMITOMO, NAKAHATA, HIDEAKI ITAMI WORKS OF SUMITOMO, SHIKATA, SHINICHI ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1994
Main classification: C23C16
IPC codes: C23C16, H03H9